The invention relates to a method for generating an AC voltage in a system frequency range from a DC input voltage in which the DC input voltage, pulse-width modulated, is connected to the primary winding of a transformer at a higher switching frequency than the system frequency.
The invention also relates to a voltage converter for converting a DC input voltage to an AC output voltage in the system frequency range using a transformer. Selectively connected to a primary winding of the transformer is the DC input voltage, pulse-width modulated via a switching device which is driven by a drive circuit at a higher switching frequency than the system frequency.
In order to generate an AC voltage in the system frequency range, for example 117 V/60 Hz or 230 V/50 Hz, from a DC input voltage of, for example, 12 or 24 V, two stages are usually carried out according to the prior art, for example as explained below:
In a first stage, an intermediate circuit voltage of 310 V is generated by means of a step-up converter from the low DC input voltage of 12 V. A flyback converter or forward converter is used for this purpose, using pulse-width modulation in a frequency range above the audible range, for example 20 kHz or more. The intermediate circuit voltage is regulated to ensure that it is constant. This high DC voltage is then converted by means of a full bridge rectifier to obtain the desired AC voltage, pulse-width modulation again being used at a switching frequency of 20 kHz or more. The envelope of the stepped voltage obtained here forms the system frequency output voltage. It is clear to those skilled in the art that this two-stage method necessarily leads to greater losses and decreased efficiency. Inductors are required in both converter stages, a transformer being required in at least one stage in order to provide the DC isolation that is usually required between the input and the output. These inductors and the required intermediate circuit capacitor are disadvantageous in terms of costs.
If the DC input voltage of, for example, 12 V is converted directly to a 50 Hz voltage then this requires an expensive 50 Hz transformer to achieve the desired 117/230 V on the output side.